quit: don't call signal() if we're using sigaction()

At best, this is a no-op.

At worst, it might:
 - Clobber a signal handler someone registered after us
 - Overwrite the signal mask or flags
 - Cause unregistration to fail (sigaction() isn't guaranteed to return the exact pointer passed to signal())

src/events/SDL_quit.c

@@ -49,8 +49,10 @@ static SDL_bool send_foregrounding_pending = SDL_FALSE;
 
 static void SDL_HandleSIG(int sig)
 {
-    /* Reset the signal handler */
+#ifndef HAVE_SIGACTION
+    /* Reset the signal handler if it was installed with signal() */
     (void)signal(sig, SDL_HandleSIG);
+#endif
 
     /* Send a quit event next time the event loop pumps. */
     /* We can't send it in signal handler; SDL_malloc() might be interrupted! */